Ordinary glasses (limesilicate or borosilicate) have been used as container materials for lamps and vacuum tubes. However, with increasingly more complex design of improved lamps types, other container materials with ability to withstand higher temperatues and corrosive chemical atmospheres are constantly being sought. Because of their impervious nature, high mechanical strength, good thermal and electrical insulating properties, high permeability to visible radiation, etc., fused silica and alumina -- the latter in densely sintered polycrystalline, translucent form, or in the form of monocrystalline alumina or synthetic sapphire -- are increasingly used as discharge vessel (arc tube) for high intensity discharge lamps and as containers for vacuum tubes.
In many high intensity discharge lamps, halides of one or more elements are added to the gas fillings to enhance the light output and to improve the color rendition as taught in U.S. Pat. No. 3,234,421 -- Reiling. In the case of fused-silica arc tubes, a vacuum-tight condition is achieved by pinch-sealing the two ends of the tube at a temperature of about 1600.degree. C with inleads of refractory metal extending through the sealed ends. The use of fused silica arc tubes for such discharge lamps, however, imposes certain constraints. Thus, several halides react with fused silica at the lamp-operating temperature resulting in leakage and ultimate lamp failure. In addition, part of the halide is removed from the discharge atmosphere through such reactions, causing a reduction in the contribution of the halide to the radiation emission. Halides which cannot be used in combination with fused silica are, for example, cadmium iodide, aluminum iodide, dysprosium iodide, and several other chlorides and bromides. Furthermore, the maximum permissible operating temperature of lamps with fused silica arc tube is around 900.degree. C. Many halides with potential for enhanced color rendition have vapor pressures too low at this temperature to be effective as arc material in fused-silica arc tubes, for example: dysprosium iodide, cerium iodide, etc.
One way to overcome the preceding problems is by selecting a discharge-vessel material which can operate at a higher temperature and resists chemical attacks by more reactive halides. A promising material is alumina in densely sintered polycrystalline form or in the form of sapphire. Alumina readily withstands a temperature of 1300.degree. C, and it is chemically resistant to many halides. Alumina arc tubes are currently used in sodium vapor discharge lamps wherein the lamp operating temperature is around 850.degree. C. Electrical current is supplied to the electrodes through inleads passing through ceramic plugs in the tube ends or through metal (niobium, tantalum, molybdenum, etc.) end caps. In either case the end closure assemblies are cemented to the alumina tube by means of sealing glass with composition based on the system CaO-MgO-Al.sub.2 O.sub.3 (U.S. Pat. No. 3,281,309 -- Ross) or on the system CaO-BaO-MgO-Al.sub.2 O.sub.3 (U.S. Pat. No. 3,441,421 -- Sarver).
In newer designs of high intensity discharge lamps now being considered, operating temperatures considerably higher than 850.degree. C are envisaged. For example, arc lamps using alumina ceramic envelopes and a fill including rare-earth halides as described in U.S. Pat. No. 3,334,261 -- Butler et al., have enhanced light output coupled with improved color rendition provided the lamp operating temperature is pushed up to around 1200.degree. C or above. Known seal glasses based on the system CaO-MgO-Al.sub.2 O.sub.3 or CaO-BaO-MgO-Al.sub.2 O.sub.3 soften at this temperature leading to leakage and ultimate lamp failure. In addition, several metallic halides, such as scandium iodide, dysprosium iodide, yttrium iodide, etc., with good promise as arc material components react with alkaline-earth oxides of the seal glass or with niobium metal of the inleads or of the end cap. This results in the sealed joint being no longer vacuum-tight, and the lamp fails due to leakage. Also, such reactions tie up halides from the discharge atmosphere and at the same time, introduce elements from the seal glass into the discharge material, causing a deviation from the designed radiation emission.
In U.S. Pat. No. 3,588,573 -- Chen et al., the thermodynamic basis for predicting reactions between the arc materials and the seal glass is outlined and a set of seal glass compositions based on the systems R.sub.2 O.sub.3 -Al.sub.2 O.sub.3 (R = Rare-earth) is suggested. The proposed seal glasses are chemically compatible with the halides in the discharge atmosphere. However, they melt only at exceedingly high temperature (around 1800.degree. C) and this means that the ends of the arc tube must be subjected to such temperatures during the sealing operation. This introduces manufacturing difficulties and causes thermal stress and mechanical tension in the seal joint area upon cooling after the sealing operation, leading to a high rate of rejects and high costs.
The object of this invention is to provide sealing glass compositions for use with high intensity ceramic discharge lamps wherein the foregoing dificulties are either eliminated, or at least minimized.